Gordon Brown and David Cameron went head to head tonight to showcase their green credentials as the two men gave keynote speeches on the environment.

The chancellor and the Tory leader outlined rival visions for cutting carbon emissions, as both shaped up to fight each other for the right to be prime minister.

The twin speeches in London came ahead of the publication tomorrow of the government's climate change bill.

Mr Brown praised last week's EU announcement on cutting emissions by 20% by 2020, and claimed that the Tories were too Eurosceptical to achieve international action.

The chancellor called for a "new world order" to combat climate change while dismissing the Tories' proposals as "ill-conceived, short-termist, unworkable and unfair". And he urged greater home insulation, energy-saving lightbulbs and an end to stand-by buttons on home appliances.

Mr Cameron, meanwhile, promised a range of new taxes on aviation - but with tax breaks for married couples and others to compensate.

While Mr Cameron has to be careful not to alienate the pro-business wing of the Tory party, Mr Brown is also operating under constraint - a wish to keep some policy surprises up his sleeve for if and when he takes over from Tony Blair as prime minister.

Al Gore, the former US vice-president turned environmental campaigner, called the political debate between the two men "a breath of fresh air" compared with the situation in the US.

Mr Gore was in London today to launch his new youth TV channel.

David Cameron this morning defended his party's plans for new taxes on airlines, saying that they would "change behaviour... not just clobber passengers".

"And once it has slipped, it's much harder to make up the difference later."

The Tories' aviation plans, outlined at the weekend, would see the imposition for the first time of VAT or fuel duty on domestic flights within the UK, and some form of "air miles allowance" for families before they started paying green taxes on further or longer distance flights.

The aim is to radically curb the number of short-haul flights.

Mr Cameron said this morning: "We are trying to look at ways where we can actually change behaviour so we can make an impact on climate change.

He told the BBC: "That is why domestic flying is important.

"There are still around 40 flights a day to Manchester although there is a perfectly good train service.

"We do need to try to have taxes in areas that would change people's behaviour and have an impact, rather than just clobbering passengers, which is what Gordon Brown did when he doubled air passenger duty in the budget."

He said that the main purpose of his proposals was to curb the growth of air transport.

"We know it's going to grow," Mr Cameron said. "Today it maybe is responsible for 5% of carbon emissions. Experts tell us it could be 25% by 2050.

"What we should be doing is taxing in a smart way that changes behaviour at the margin and tries to curb this growth. We're not putting on a hair shirt and saying to people 'No more holidays'.

"That's absolutely not what we are doing. These are all replacement taxes, not additional taxes," he added.

With some of the Tory press already outraged at Mr Cameron's suggestion, he insisted that the additional tax take would be counteracted by tax cuts elsewhere.

In a round of interviews this morning, he told BBC Breakfast: "What we have said is any green taxes we introduce will be offset by cutting taxes on families, or on business, one for one."

Mr Cameron added that the Tories had said they would like to have a tax break for married people and one of the ways they would fund this would be through green taxes.

The Tory consultation paper, Greener Skies, also puts forward the option of fuel duty or VAT - or both - on domestic flights.

The package of proposals includes a "green air miles allowance" giving people one short-haul flight a year at the standard rate of tax before the higher rates kick in.

Other measures include a proposal to replace air passenger duty with a new per-flight tax, based more closely on actual emissions, which would see aircraft which are less fuel efficient, or which fly half-empty, taxed more heavily.

But Mr Cameron was forced to distance himself from comments made by John Redwood, the former Tory minister, who has declared that he is "sceptical" about global warming.

In his blog, the MP for Wokingham also wrote that if climate change is happening, it could have some beneficial effects.

"I'm told that John Redwood's comments on his blog are sort of a jolly aside," Mr Cameron insisted.

The Green party today picked the woman behind the anti-urban 4x4 campaign, Sian Berry, as their candidate to be London mayor next year.

Ms Berry, the party's current national speaker, said she would fight for a "city run on a human scale, which enriches the lives of everyone".